Telephone repeater



A ril 9, 19535. L. e. ABRAHAM a" AL. 1,997,245

TELEPHONE REPEATER Filed Sept. 1,.1933 2 Sheets-Sheet 1 it I .Wtworlc INVENTORS ATTORN EY April 9, 1935.

L. G. ABRAHAM ET AL TELEPHONE REPEATER Filed Sept. 1, 1955 JVE'tLogrk 2 Sheets-Sheet 2 INVEN'II'ORS ATTORNEY Patented Apr. 9, 1935 1,997,245 TELEPHONE REPEATER' f Leonard G. I,Abraham, Madison, and Doren Mitchell, Bound Brook, N. J assignors to @American Telephone and Telegraph Company, 7

a corporation of New York Application Septem r 1, 1933, Serial Noteszsso ciolaims." (01.179-170) h This invention relates to telephone repeaters, and more particularly to two-wayg'repeaters for use with irregular two-wiretelephone lines.

It is the object of theinvention to provide a 5 typeofrepeater that can be'usedwith. lines of such a degree of irregularity. in construction as to preclude the use of ordinary types ofrepeater with more than a very low gain. 'Asecond object of the invention is to obtaina'substantial gain 10 from such a repeater, while introducing a minimum of distortion or other disturbance in the telephone circuit. Y e 1 e .A further objectof the invention is to'provide means-for preventing noise whichmay exist on the telephone circuit from interfering with the obtaining of the desired results from the repeater.

, It is well known that with the type of telephone.

repeater ordinarily used with a two-wire line, the amount of gain that-can be obtained is lim: ited by the similarity between the impedences of the lines in the two directions and those .of the artificial linesor networks thatareused to balance them. The measure. of this similarity is called the return loss between the line and its network. This similarity in turndepends largely upon the uniformity or regularity with whichthe lines are constructed, since, while it is usually 7 possible to construct a simple network whose'impedance is similar to-that of a line of uniformly distributed or regularly recurring constants, it is usually .not possible to, construct at reasonable cost a similar network for an irregular line, This is particularly true of a loaded line. If the'loading coils are of equal inductance and, equally spaced, and the capacity of the-line itself is-uniform, little difliculty is encountered in constructing a simple network whose impedancecharacteristic issimilar to that of the line, within the frequency range that is of interest; If there are serious irregularities in inductance or capacity, however,, or in the spacing of the coils, the impedance characteristic of'the line isso, irregular that itis impossible to simulate it with any reasonable type of network. a

It is usual, therefore, to make every efiort'to secure a high'degree of uniformity in constructing a line with which repeaters are to be used. Sometimes, however, this is impossible to attain; or it may become necessaryto use repeaters on without thought of suchuse. Under these circumstances the 'impedanceof the line may be so irregular as to make it impossible to obtain the desired gain from the repeater. Again, it may be desirable to switch a repeater to any one of livered to coil C. a line which-had 'previouslybeen constructed severallines of diiferent impedances. In such a'case it would be necessary" to use a network 5 whose impedance, was a compromise between those of the'various lines, and it'would not be possible to obtain'alhigh singingpoint against any of themj It iss'uch conditions as'thesethat can be taken care of by our invention. i

The 'efiect or, impedance dissimilarity oi'jl'ow return loss between the lines and their respective networks is to cause circulating currents to'fiow within the repeater, thus interfering with its action in amplifying telephonic currents-arriving from the lines. We'propos'e to insertin the'path of these circulating currents sufiicient'attenuation to reduce them tojnegligible proportions, and to provide Voice-operated devices, actuated by the telephonic currents arriving from the lines, to divide the attenuation unequally between the. two

sides of the repeater; Thus whentelephdnic cur- I rents reach the-repeater, forexampleiromthe west line, themajor partof the attenuation will be inserted in the sideof' the repeater transmitting from east to west, ,sothat' the telephonic currents passing through fromwest. to east will receive a net amplification. j Qf The invention will be clearly understoodjwhen the followingdescriptionis read withreierence to the accompanying drawings." In the drawings, Figure 1 shows one embodimentof the in?- vention in its simpler form, while Fig. 2 shows the inventionin a'somewh'at more complex ar rangement which is deemed preferable-in certain circumstances. Similar reference characters {designate similar parts of the circuit inthe two figures of the drawings.

In' each figure of the drawings,.A and A dicate the telephone lines in the two directions;

and B and B the artificial lines intended to balance them. C and C" are'balancedjor' hybrid coils .or transformers asordinarilyhsed in two'- known form of two-wire telephone repeater.

With this type of repeater, voice currents'arriving over the line A are amplified in D and: de- If line Aand network B are approximately equal in impedance, theamplified voicepower will divide approximately equally between them, and only a smallportion of it will pass through' to conductors'J and enter amplifierD. The portion of the "power' 'which thus passesthrough'the coil from Kto J is a wire repeaters, and D and D are the amplifier cir- 'cuits for the two directions of transmission. Thus, B, C, D and B, C, and D, as connected function of the return loss between A and. B.

The power enteringamplifier D will be amplified and delivered to coil C, and a portion of it, determined' by the return loss between A and B, will reach conductors J and be amplified again by D. If the sum. of the two amplifications in D and D is greater thanthe sumjof the two losses in passing through coils C and C, a circulating current will be sustained through the repeater, and it will become practically inoperative for transmitting conversation.

To avoid this condition, and permit ayreason able gain to be obtained even when the return loss is low, we propose to insert losses in the path of the circulating currents, which losses are under the control of the voice currents themselves. These losses are designated in Fig. 1 by H and H: they are indicat'edas simple shunt re sistances, but it will be understood that .theymay take a variety of forms, andthat their position in the. circuit may be changed, without departing detectorsare bridged across the conductors J and J7, respectively, in parallel with the inputs of the repeater amplifiers. D and D- They arethere- .fore affectedby incoming speech currents from the lines A and A,-respectively.

Consideringthe effectof speech currents-arriving from line A,-it will be seen that a portion of themwill enter amplifier-detector E, and if of sufficient magnitude will cause relay F to oper ate. The upper contact; of this relay closes the circuit through the winding of relay G, which re-' .moves loss H from the transmission circuit, thus permitting thegain of amplifier D to be effective. The lower contact of relay F breaks-the circuit from amplifierdetector E to relay F, thus making it impossible-for relay F tooperate, and ensuring thatloss I -I'sremains in the path of the circulating currents to prevent singing. It .is essential that either H or H shall be in the singing .pathat all times. I V l r I When speechcurrents cease to arrive from line A, relay Freleases, breaking the circuit through relay G, and restoring -the connection-between E and F. Loss H is thus reinserted, and both amplifier-detectors are in a position to function. Relays F and F may be arranged for slow' release, so that they will not drop back between syllables or between words, but only during definite pauses in'the'speech.

On the arrivalof speech currents over line A, an exactly analogous operation takes place through amplifier-detector E and relays F. and G. .Loss'I-I is removed from the circuit,.and amplification is provided for the speech from' A towards A. Operation of relay F is prevented by the open contact of F, and loss His maintained in the singing path. Each direction of conversation thus receives amplification, in spite of low return losses. I

If speech currents arriving from either line are. too weak to operate the relays, losses will re- .malinin the circuit, and such currents will not receive-the desired amplification. They will not be lost altogether, however, since losses H and H will be made only just large enough to provide a suitable margin of safety against singing, and against the distortion which results from a near approach to singing. It may even be possible to obtain some amplification, if the return losses are not too low.

The simple circuit of Fig. 1 is, under certain conditions, open to a number of objections. We

therefore prefer the circuit of Fig. '2, which ac- .complishes' essentially the same result, but in an improved manner. 7

One of the important objections to the circuit of Fig. lie that each of, the losses H, H must be great enough in itself to provide-a suitable margin against singing. In the normal or unoperated position of the relays, both losses are in the circuit, giving a much greater margin than necessary. Weak speech, and the initial sounds of speech before the relays have time to operate, will therefore be attenuated approximately twice as much as is needful. In the preferred circuit of Fig. 2 we have avoided this effect by dividing each loss into two parts, H1, H2, H1 and'I-Iz", respectively, controlled'by the relays G1, G2,. G1 and G2". Any two of these losses are sufficient to give the desired'margin against singing. In the normal position of the relays, H2 and H2 are in the circuit, and Hi -and H1" are disconnected; Thus weak speech in either directionsuffers only half the loss imposed by the arrangement'ofFi'g. 1. On the arrival of speech from line A, loss H2 is removed from 'the'circuit, andloss H1 is'simultan'eously inserted in the other side, thus maintaining the singing margin, but obtaining the full gain of the amplifier '-D in the direction of speech. -'Similarly, the arrival of speech from line A, with the relays in the normal position, will remove :loss H2 and insert loss Hi. The manner of accomplishing this will appear fron'ia detailed description'of the operation of thecircuit. r 1 I I With both relays F and F" normal, their contacts serve to short-circuit the windings off relays G1, G and L, and G1, G2 and L, respectively. 'On'th'e arrival of speech power from line A, rectified current from amplifier-detectorlil will operate relay This same current will pass through a biasing winding M on relay F, the direction of this winding being such as to oppose the operation of relay F. The purpose of this is to prevent false operation of this' rel'ay by speech-currents arriving from line A, being amplified by. amplifier D, and passing through coil C and on o'ver J' by reason of the unbalance between line A and its network B. Such false o'p- -eration might occur with the simple circuit of Fig.

1. As soon as relay F operates; current will flow from battery "Q, through contact P of relay- L, through resistance R, and since the ground through the contact of relay F has been removed, through the windingsofrelays G2, L and G1. Relay G2 removes loss H2; relay 'Gi inserts loss 'Hi;

theopenin'g'ofcontac'tl? "of relay L disables the 7 may be noise currents, either induced in the line F, G1, G2 and L will release. after a short interval determined by their construction. 'H1will then be removed from the circuit and H2 inserted, and the device will be ready to operate on speech from either direction. 0n speechvcurrents from line A the action is exactly analogous, loss H1 being inserted by relay G1, .loss 1H2 being removed by' relay Gzflthe'other'relay chain beingxdisabled by the removalof battery Q through contact P'' of relay L, and biasing winding M of relay F be,- ing short-circuited'by contact. N of relay :L.

There isone diificultythatamay at times be encountered in'the operationofthe device as described to this point; It is desirable to make the relays and the amplifierf-detectors quite sensitive, -in order to assure as far as possible that weak speech .will beable to" operate the. relays and obtain the gain of the repeater. Weak speech, of course, particularly needs the amplification. On the otherhand, with this high sensitivity, there or picked up by the subscribersfitransmitters, which will be of sufficient magnitude to cause the relays to operate. If the noise arrives. steadily from one direction, the-device will'remain operated in that direction, and it will be impossible for speech currents from the other direction to obtain amplification. I

To avoid this eifect, where necessary, there may be added to the arrangementalready'described a device for automatically reducing the sensitivity of the relays when noise is present." This device is essentially the same as that disclosed'in U. s. Patent No. 1,772,551, granted to D.

Mitchell and H. C. Silent. It consists of an auxiliary amplifier-detector circuit, designed to be particularly sensitive to alternating currents of approximately constant intensity, such as would be caused by steady noise, but much less sensitive to currents whose intensity varies rapidly through a wide range, like those caused by speech. The output of the amplifier-detector is used to provide an additional bias opposing operation of the relays, thus reducing their sensitivity in the presence of noise to a point where the noise is'incapable of operating them; This reduced sensitivity will, of course, result in loss of amplification on an increased percentage of the speech currents; but this is probably less objectionable than the possibility of locking up the circuit due to noise. The full sensitivity will be obtained when no noise is present.

In Fig. 2, S designates the amplifier-detector circuit, substantially as described in the patent already referred to. It is connected to the circuit through a hybrid coil T, similar to C and C. The two branches of the hybrid coil are connected to the conductors J and J, in parallel with the inputs of amplifier-detectors E and E, respectively, through hybrid coils W and W. U' is a resistance so proportioned to the input impedance of the amplifier-detector S that there is substantially no transmission through the coil T from J to J or vice versa, approximately half of any power from either J or J being dissipated in U, and the balance entering S. A portion, therefore, of any speech or noise currents reaching the repeater from either A or A will enter the amplifier-detector S. Speech currentswill produce comparatively little effect, since "as already explained, the circuit is designed to be relatively insensitive to them. iNoise currents, however, will produce a direct current in the output circuit of S, the magnitude of this output current depending on the intensityof the noise.

The output'current passes through the auxiliary biasing. windings V' and V in series, the direc-- tions of-these' windings being suchas to oppose the operation of the respective .relays F. and F.

The proportioningiof theicircuit is such that'this I bias is sufiicient-to preven the operation'of the relays due to steady noise;. They will" stillzoperate on the moreintense portions of speech currents, though with reduced sensitivity. This Joperation issubstantiallyrthe same as that described in the patent alreadyre'f erred to, 'inl'connection with'echo suppressors. Y a

Thepurpose *ofhybrid coils W and W 'is to prevent the insertion of losses H1 ,or H1 from immediately and when the lossjwas removed the -noi'se"'would then be able; to" operate relay. F- or F,'as' the case-might be,-for anappreciable in- 'terval until the biascurrent built up again.' This would result in 'objectionable mutilation of speech. ThenetworksX and-Xare of approximately the same impedance as thatof the lines J 1 'and'J 1',

respectively. This .r'neans "thatrwhen energy from line'A' is reaching 'coil T a change in impedance in line J2 has very little effect on this energy. The same is true of energy passing from line A to coil T.

1. In a telephone repeater for use with twowire telephone lines,'two separate amplifying elements with variable gain for transmission in the two directions, a voice-operated device associated with each amplifying element, a relay controlled thereby, eachrelay being adapted to increase the gain of the associated amplifying element and to make a corresponding decrease in the gain of the other amplifying element, and

means controlled by each voice-operated device for reducing the sensitivity of the relay controlled by the other voice-operated device. 7

2. In a telephone repeater for use with twowire telephone lines, two separate, amplifying elements with variable gain for transmission in the two directions, a voice-operated device asso ciated with each amplifying element, a relay controlled thereby, eachrelay being adapted to increase the gain of the associated amplifying element and to make a corresponding decrease in the gain of the other amplifying element, means controlled by each voice-operated device for reducing the sensitivity of the relay controlled by the other voice-operated device, and means for preventing circulating. currents from in turn. reducing the sensitivity of I either of the relays once it has operated. v

3. Ina telephone repeater for use with twowire telephone lines, two separate amplifyingele- ,ments'with variable gain for transmission in the two directions, a voice-operated device associ--' ated with each amplifying element, and a relay system controlled by each voice-operated device, each relay system being adapted to increase the gain of the associated amplifying element, to

make a corresponding decrease in the gain. of the other amplifying element, and to prevent the operation of the other relay system, the repeater being at all times operative to conduct current in each direction to an appreciable extent.

14. In a telephone .repeater'for use with twowire telephone lines, two separate amplifying elements with variable gain for transmission in the two directions, a voice-operated device associated-with each amplifying element, a relay con-' trolled. thereby, each relay being adapted to increase the gain ,of the associated amplifying element and tomake a': corresponding decrease in thergain of the other amplifying element, and means responsive to noise in either side of the circuitIfoi biasingbothrthe said relaysto; prevent operation thereof by the noise, the repeater being'at all-timesroperative 'to conduct current in each direction to an appreciable extent. I

5. In a telephone repeater for use with'twowire telephone lines, two separate amplifying elements with variable gain for transmissionin,

the two directions, a voice operated device associated :with each amplifying element, a relay controlled thereby, each relaybeing adapted to increase thegai-n of therassociated amplifying element and to make ,a corresponding decrease in the gain. of the other amplifying element, means responsive to noise either side of the of the other amplifying element, means responsive to noise in either side 'of the circuit for biasingboth the :said relays to prevent operation thereof by the noise, means for preventing the biasing of the relays in response to voice waves,

"and means for preventing interaction between the two sides of the repeater due to the association'therewith-of the biasing means, the repeater being atall times operative to conduct current in ea'ch'di-rection to an appreciable extent.

7 LEONARD G. ABRAHAM.

DOREN MITCHELL. 

